You are writing a reusable library, and you do not know where or how your code will execute. You need to write log messages to an abstracted logging interface because you cannot count on the presence of Log4J or JDK 1.4 logging.
Write messages to the Commons Logging Log
interface, and rely on Commons Logging to decide which
concrete logging framework to use at runtime. The following code uses
the Log
interface to log trace,
debug, info, warning, error, and fatal messages:
import org.apache.commons.logging.LogFactory; import org.apache.commons.logging.Log Log log = LogFactory.getLog( "com.discursive.jccook.SomeApp" ); if( log.isTraceEnabled( ) ) { log.trace( "This is a trace message" ); } if( log.isDebugEnabled( ) ) { log.debug( "This is a debug message" ); } log.info( "This is an informational message" ); log.warn( "This is a warning" ); log.error( "This is an error" ); log.fatal( "This is fatal" );
LogFactory.getInstance( )
returns an implementation of the Log
interface, which corresponds to an underlying concrete logging
framework. For example, if your system is configured to use Apache
Log4J, a Log4JLogger
is returned,
which corresponds to the Log4J category com.discursive.jccook.SomeApp
.
The developers of a reusable library can rarely predict where and
when such a library will be used, and since there are a number of
logging frameworks currently available, it makes sense to use Commons
Logging when developing reusable components such as Commons components.
When LogFactory.getInstance()
is called, Commons Logging takes care of locating and
managing the appropriate logging framework by testing a number of system
properties and libraries available on the classpath. For the developer
of a small reusable component, the complexity ends at the calls to the
Log
interface; the burden of
configuring the underlying logging framework is shifted to the developer
integrating this library into a larger system.
Recipe 7.11 details the algorithm Commons Logging uses to identify the appropriate concrete logging framework to use at runtime.